This application is based on Japanese Patent Application No. 2002-136955 filed on May 13, 2002, the disclosure of which is incorporated herein by reference.
The present invention relates to a vapor compression refrigerant cycle using refrigerant having a critical temperature equal to or less than sixty degrees Celsius, such as carbon dioxide, and suitably used for a vehicular air conditioner.
In a vapor compression refrigerant cycle, as is well known, high-pressure refrigerant compressed in a compressor is cooled in a high-pressure side heat exchanger and low-pressure refrigerant in a low-pressure side heat exchanger is evaporated, so that heat from low-temperature refrigerant is transferred to high-temperature refrigerant.
For example, if a vehicle has been left under a burning sun for a long time in summer, temperature in a passenger compartment increases more than sixty degrees Celsius. In this case, therefore, a vehicular air conditioning unit requires capacity to quickly cool the inside air to approximately twenty-five degrees Celsius.
During such a quick cooling operation generating a large heat load, pressure of refrigerant in a low-pressure side heat exchanger, that is, in an evaporator, increases. In a case that refrigerant having a critical temperature that is higher enough than a temperature of air to be cooled, such as flon (R134a), is used, the temperature of the low-pressure side refrigerant falls less than the critical temperature even if the heat load is large. Therefore, because the low-pressure side refrigerant is in a gas and liquid phase state, air to be blown into the passenger compartment can be cooled by evaporating the refrigerant. Here, R134a has the critical temperature of approximately one hundred degrees Celsius.
On the other hand, in a case that refrigerant having the critical temperature that is lower than the temperature of the air to be cooled, such as carbon dioxide, is used, if the heat load increases, the temperature of the low-pressure side refrigerant is likely to increase equal to or higher than the critical temperature. As a result, the pressure of the low-pressure side refrigerant may increase equal to or higher than a critical pressure. Here, carbon dioxide has the critical temperature of approximately thirty-one degrees Celsius.
When the pressure of the refrigerant is higher than the critical pressure and the refrigerant is in a critical state, there is no physical difference between gas refrigerant and liquid refrigerant. Therefore, latent heat of vaporization of the refrigerant becomes substantially zero, so the air to be blown into the passenger compartment is cooled only by sensible heat. As a result, as compared with a case that the air is cooled by latent heat of vaporization by vaporizing the refrigerant, cooling capacity (refrigerating capacity) of the evaporator is largely decreased.
Also, in a vapor compression refrigerant cycle having a gas-liquid separator at an inlet side of the compressor for separating the refrigerant into gas refrigerant and liquid refrigerant, if pressure of the low-pressure side refrigerant increases equal to or higher than the critical pressure, pressure inside the gas-liquid separator also increases equal to or higher than the critical pressure. Therefore, it is difficult to separate the refrigerant into the gas refrigerant and the liquid refrigerant in the gas-liquid separator.
Further, in a vapor compression refrigerant cycle having an ejector (see JIS Z8126 No. 2. 1. 2. 3) as a pumping means for circulating the refrigerant to the evaporator, the refrigerant is supplied to the evaporator from the gas-liquid separator provided at the inlet side of the compressor. Therefore, if the refrigerant that has been heated in the evaporator flows in the gas-liquid separator, the refrigerant in the gas-liquid separator is heated and therefore the temperature of the refrigerant to be supplied to the evaporator increases. As a result, the cooling capacity (heat absorbing capacity) of the evaporator further decreases.
The present invention is made in view of the foregoing disadvantages and it is an object of the present invention to provide a vapor compression refrigerant cycle using refrigerant that has a critical temperature equal to or less than sixty degrees Celsius, which is capable of restricting a large drop in refrigerating capacity when the large refrigerating capacity is required.
According to the present invention, a vapor compression refrigerant cycle for transferring heat from low-temperature refrigerant to high-temperature refrigerant includes a compressor compressing refrigerant, a first heat exchanger for cooling high-pressure refrigerant compressed in the compressor, and a second heat exchanger for vaporizing low-pressure refrigerant after being decompressed. The refrigerant has a critical temperature equal to or lower than sixty degrees Celsius. In the cycle, a pressure of the refrigerant in the second heat exchanger is controlled equal to or lower than a predetermined pressure.
By controlling the pressure of the refrigerant in the second heat exchanger, a large drop in cooling capacity is restricted.
Preferably, the pressure of the refrigerant in the second heat exchanger is controlled by controlling pressure of the low-pressure refrigerant with a controlling device.
Preferably, the pressure of the refrigerant in the second heat exchanger is controlled by controlling a flow rate of air passing through the second heat exchanger. Because a heat exchange rate in the second heat exchanger is controlled, the pressure of the refrigerant in the second heat exchanger can be maintained below a critical pressure. Alternatively, the pressure of the refrigerant in the second heat exchanger is controlled by controlling a flow rate of the refrigerant to the second heat exchanger.
Preferably, the cycle further includes an ejector for decompressing the refrigerant discharged from the first heat exchanger, a gas-liquid separator for separating the refrigerant into gas refrigerant and liquid refrigerant, and a valve provided on a refrigerant passage connecting the ejector and the gas-liquid separator to bypass the second heat exchanger. The valve closes to block the refrigerant passage when the pressure of the refrigerant in the second heat exchanger is higher than the predetermined pressure that is equal to or lower than a critical pressure of the refrigerant.
Accordingly, the low temperature refrigerant after compressed is directly introduced into the second heat exchanger. Because the refrigerant to be supplied to the second heat exchanger is maintained at low temperature, the large drop in the cooling capacity is suppressed.